I can't sign in with my Google Account username and password

You're not using the correct Google Account. Sign in with the Google Account you specified during the update process.

You still need to update to a Google Account. Sign in with your DoubleClick username and password, or click the Update Now link to start updating your sign-in information.

You're on the DoubleClick sign-in page instead of the Google sign-in page. Click the Sign in with your Google Account link and enter your Google Account sign-in information on the page that appears.

You're already signed in to another Google Account on this computer. You can be signed in to only one Google Account at a time. Sign out of the other Google Account, and try to sign in to your DoubleClick product again.